William Cumpiano's hardware-based neck joint

I devised this joint as an alternative to the more complex and challenging pinned mortise and tenon joint described in my guitarmaking book.

This new hardware-based neck joint system relies on a common brass hardware-store fastener called, variously, a "barrel and bolt" , "KD (for knockdown) fastener," or "connector bolt and dowel."

 Any good hardware store carries them. If you don't have a good hardware store nearby, here's where I get my "knock-down bolts" (part no. 36250) and "knock-down nuts" (part no. 38220) on the web: click here.

Why is this alternative necessary?

My guitar-making textbook was first published in 1985, and has gone into six editions without a major revision. Well, its time. This is it: but instead of republishing an entire edition, I'm just posting the first major revision here.
    Over the years, I've received quite a bit of mail from amateur builders struggling with the pinned mortise and tenon neck-joint system detailed therein. Its major drawback: the pinholes in the neck tenon must be offset by a minute amount to snug the neck down and into the soundbox. So how much is "minute," exactly?

I learned the system in the Gurian workshops during the early seventies. The system was reportedly adapted by Michael Gurian and Walter Lipton from post-and-beam barn construction. In this ancient joint, oak pins are hammered through holes in the mortise into holes holes drilled in the tenon, holes drilled slightly offset, thus drawing the two tightly and permanently together. On the barn, both mortise and tenon were massive, and the pin was knocked in with a hammer.

The joint's guitar counterpart requires much greater finesse and precision. An offset of one to two sixty-fourths of an inch is correct: much more and the pin simply refuses to enter the tenon or, if it does, mayhem can ensue: the neck shaft can be drawn forcibly away from the fingerboard. This rarely happened at the Gurian shop: we all learned how to install it without problems.

But, alas, hindsight has shown that it is not a good neck joint to learn to do from a book. I've become aware over the years that although some seemed to handle its challenges pretty well, other people using were stymied by the precision required for it to work properly, if not the number of special gadgets, fixtures and implements that have to be fashioned to handle its assembly.

So what are the alternatives? The tapered dovetail joint? Heavens, no! If there was ever a joint designed in hell, that one is it! There is little to be said for the joint to recommend it to hand builders. For us, the tapered dovetail is unnecessarily complex, requires endless fussing to get it to fit properly and as well, to disassemble it for resetting.

So here we have a joint I like and can recommend--AND am using exclusively myself. I have put down my pinning pliers forever! For those purists who maintain that hardware-based joints are untested Johnny-come-latelies, allow me to refer you to Renaissance lutes which often had a large, single wood screw holding the necks on. They've been around at least as long as mortise and tenon joints, tapered or straight.

I'm very pleased with the barrel bolt: It works remarkably well: its strength is more than adequate to the requirements; it does not budge, even after only gently tightening it. It is SO easy to construct, assemble and disassemble. Anybody want to debate the merits/demerits of this system vis-a-vis any other? I'll post it here.

 

 

I think the drawing is self explanatory. Drill the barrel and bolt holes in the neck blank on the drill press while it's square BEFORE sawing the tenon and shaping the heel.

Headblank-drilled-for-hardw.jpg (7845 bytes)

The hole in the tenon for the barrel should be located as close to the back surface of the heel ("the third line") as possible, leaving as much tenon as possible remaining in front of the barrel for strength. Drill the barrel holes in the side of the blank (eventually, the side of the tenon) so the barrel fits tightly (I purchased a millimeter size drill to match mine perfectly). Drill the holes for the bolt over-size so the system will accommodate misalignment (in the headblock, 3/8"; in the end of the tenon, 5/16") . You may have to hacksaw or grind the barrel's length to the width of the tenon. Hacksaw a slot into the end of the barrel so that you can rotate it with a screwdriver while its in the tenon when you align the bolt to it. You must align the bolts to the barrel and insure that the threads are mating before attempting to actually bolt the neck to the body

When ready to attach the neck to the soundbox, apply glue under the fingerboard, throw a padded cam-clamp lightly from the fingerboard extension across the box to the back in order to snug the fingerboard down to the body (do several dry runs, of course to optimize fit and neck angle). THEN insert the bolts via the soundhole into the holes in the headblock and screw the neck into the body. Tighten the top bolt first, then the bottom. The remove the clamp, slip the fingerboard end caul and upper face-brace caul in place and clamp the fingerboard end down as described in the book text.

A long-handled 5/32" hex Allen head wrench can be manipulated through the soundhole to tighten the bolt. Finger-tight is quite sufficient. Don't bully it or crunch it down: not only is this unnecessary, but it may conceivably break the tenon. One more point: If you fear your customers might mistake the bolt heads for a truss rod adjusters and start to crank them down, warn them or inlay the bolt head and lightly tack a "cap" of matching wood over them to keep them from prying eyes.

By the way, I've actually converted a pinned mortise and tenon neck and headblock to this system, aftermarket, so to speak: I made a little drilling jig to slip over the tenon to guide the drills for the barrel and bolt holes, and then spotted and drilled the 3/8" holes in the headblock through the headblock mortise. If you exit slowly, there will be minimum chip-out where you exit the headblock, and any chip out will be covered by the bolt's large head.


The photo above shows neck tenon with barrels inserted and bolts threaded into them through holes in the end of the tenon. Clearly, you must removed the bolts to insert the neck into the body, and then insert and thread the bolts into the barrel after reaching through the soundhole and inserting the bolts through holes in the headblock  You can find more comments on the truss rod cap and the hard-to-find truss rod bolts here.

NEW! FURTHER IMPROVEMENT...

I discovered that my old partner Michael Millard of Froggy Bottom Guitars has been using his own version of this system, but has for good measure been reinforcing the tenon with two hardwood slats at either side, with their grain traveling at 90 degrees to the short grain of the tenon. This seemed like a splendid idea, that could update my technique easily. So here it is: new and improved.



Here's how I do it. I prepare two even grained hardwood slats, 3/32" thick (old unmatched side blanks from my scrap bin are my source). When I make the two vertical table-saw cuts for the tenon, I plan for a total tenon width to remain that is 3/32" narrower on either face than the finished, reinforced tenon. Now, my table saw blade naturally cuts a kerf that is .15". That is ideal because the slots come out .04" wider than the reinforcing slats. That allows minimal clearance so I can slide the slats in with medium-viscosity cyanoacrylate glue covering just the side that faces the tenon.
     I then proceed to drill the neckblock in the manner described above, and rout out the truss rod slot. When the time come to expose the tenon, I bandsaw "the third line", stopping just as the blade kisses the hardwood slat on both sides. The neckblank appears as it does immediately above.


"YES BUT IS IT STRONG ENOUGH?"

MarcC18642@aol.com  wrote:

> I was a bit concerned about the strength of the wood surrounding the barrels in the tenon. So I soaked the tenon wood in thin CA for a bit of strength insurance. Anyway, to make a long story shorter, I decided to see how strong this set up was. So I clamped the whole neck asembly, hardware and all, to a block of mahogany and
reefed it into my big vice. Then I sat on the headstock to see if I could make fail. It held my entire weight- 220 pounds (actuall about 250 foot pounds of rotating torque)!


William R. Cumpiano replies: This is quite a testimonial given that the actual stress across the joint is applied along the pull of the strings. So if you do a vector diagram of the forces acting on the joint, you'll find that most of the stress is compression of the neck INTO the guitar, and only a minor portion is tension actually acting on the bolt and the wood surrounding the dowel. So it appears to be far stronger than it needs to be.

Russell Rose of Avandel Guitars wrote:
russell.rose@avandel.co.uk

>You were kind enough to give me some advise and Information on your bolt on system. I have tried this on two prototype guitars and believe it to be the best bolt system I have ever seen. I carried out some crude strength test and was able to shatter (an old discarded) Mahogany neck with no damage to the bolt on joint. I just wanted to write to you and thank you, I also wanted to check if it is ok for me to use this system on the guitars that I sell? I am a small builder recently established in the UK.

William R. Cumpiano replies: Yes of course, you can use it, it's my gift to the world. I take no credit for being the first inventor anything it occurs to me to do. Too many times, I've done so, and later found someone had already thought of it, and had been doing it for years. If you want to repay me in some way, just acknowledge where you learned it from, whenever appropriate.